Field of the Invention and Related Art Statement
The present invention generally relates to a method of detecting defects formed inside a pneumatic tire in the non-destructive manner, and more particularly relates to a method of detecting rubber belt separations formed in the vicinity of an outer surface of a used tire whose tread surface has been buffed off in order to manufacture a renewed tire or in the vicinity of a smooth outer surface of a special tire such as racing tire.
In case of manufacturing a renewed tire, a tread surface of a used tire is first ground off to obtain a substrate tire having a smooth surface, and then a new tread rubber is applied in the substrate tire. In order to keep the desired property of the renewed tire, it is necessary that the substrate tire does not include large separations formed between laminated rubber belts. In order to detect the separation, it has been practiced to monitor the outer appearance of the substrate tire or hit the substrate tire with a hammer. However, this checking method requires the great experience of an operator and sometimes undesired separations might not be detected correctly. Therefore, it has been desired to develop a method of automatically detecting defects in the tire in the non-destructive manner.
There has been proposed the following methods of detecting the separations in the pneumatic tire in the non-destructive manner.
(a) Method of utilizing holography or cialography PA1 (b) Method of utilizing an ultrasonic defect detector PA1 (c) Method of utilizing displacement sensors
In the first method (a), a number of images of a tire surface are taken and defects in the tire are detected by processing these images usually with the aid of a computer. In the second method (b), the ultrasonic wave is transmitted through the tire to obtain a cross sectional construction of the tire and defects are detected by analyzing the cross sectional construction. The third method (c) is disclosed in copending Japanese Patent Laid-open Publication, Kokai sho No. 62-298,744 laid-open on Dec. 25, 1987. In this method a pair of roller pushers are urged against an inner wall of a tire and displacements of the roller pushers in a direction perpendicular to the tire surface are measured. Defects in the tire are detected by comparing the thus measured displacements with each other.
In the first and second known methods, the apparatus for detecting the defects is liable to be complicated in construction and large in size, so that the apparatus becomes quite expensive. The first method (a) has a relatively high detecting precision and a small defect having a diameter of about 5.about.10 mm can be detected, but it requires a skill of an operator. Further, the operation of detecting the defects is very cumbersome. In the second known method (b), the operation is easier than the first method (a), but has a lower precision. That is to say, defects having diameters smaller than 30 mm could not be effectively detected. Further, in order to remove the influence of ambient noise, the measuring space must be shielded, so that an extra cost is needed.
In the third method (c), since the roller pushers are urged against the inner surface of the tire, it is difficult to detect defects formed in the vicinity of the outer surface of the tire. It has been found by experience that defects are liable to be generated not in the vicinity of the inner surface of tire, but in the vicinity of the outer surface thereof. Further, since the roller pushers are pressed against the inner surface of the tire, the tire could not be inflated. Therefore, the tire is deformed to a great extent, and thus the displacement of the pushers is largely influenced upon the deformation of the tire itself and the signal-to-noise ratio of the detected signal becomes very small.